The invention is in the general field of nucleic acid hybridization.
Nucleic acid hybridization is an important technique for detecting the presence of particular sequence information. Recent advances have allowed for the production of high density oligonucleotide arrays which may have great utility for research and clinical diagnostics. Typically, a sequence is amplified and labeled with fluorescent tags. The sample is incubated with the probe array and the sample is washed with a series of increasingly stringent buffers. Stringency may be provided by means of reduced salt concentration or increased temperature. Reduced salt concentrations lead to electrostatic repulsion between phosphate groups, thereby lowering the melting temperature (Tm).
The number of different sequences on each array is ultimately limited by the detection capabilities of the instrument analyzing it and factors such as contaminating fluorescent compounds, stray light, and hybridization of mismatched oligonucleotides. As the stringency of hybridization is increased, the noise level from mismatches is reduced (increasing specificity), but the overall signal is also reduced due to loss of correctly hybridized molecules (decreasing sensitivity).
Two related and important amplification techniques which depend on the specificity and sensitivity of oligonucleotide hybridization are polymerase chain reaction (PCR) and ligase chain reaction (LCR), which are commonly used in medical diagnostics and research.
The invention relates to controlling the specificity, sensitivity, or selectivity of nucleic acid hybridization procedures. Control of hybridization according to the present invention is achieved through application of high hydrostatic pressure. Without intending to be limited to any mechanism, it is believed that increased pressure favors nucleic acid hybridization (i.e., reversible or irreversible hybridization). Regardless of the mechanism involved, high pressure increases the sensitivity, specificity, or selectivity of nucleic acid hybridizations.
Accordingly, the invention features a method of hybridizing a first nucleic acid to a second nucleic acid at least partially complementary to the first nucleic acid by (1) providing a sample vessel and pressure controller for the vessel; and (2) contacting the first and second nucleic acids within the vessel at a pressure above ambient pressure (e.g., above 10,000 psi) which is effective to enhance hybridization of the first and second nucleic acids. This method opitionally includes cycling pressure in the vessel between a first higher pressure at which the first and second nucleic acid are hybridized and a second lower pressure at which the first and second nucleic acid are denatured. In case of pressure cycling, can further include providing a temperature control for the sample vessel, and cycling the temperature between a lower temperature and a higher temperature, such that the first and second nucleic acids hybridize at the first pressure and lower temperature, and such that the first and second nucleic acids denature at the second pressure and higher temperature. Alternatively, the vessel can be maintained at a constant temperature as the pressure is cycled. Such methods can be used to amplify a portion of the second nucleic acid. An optional step in any of the above methods includes washing away unhybridized nucleic acids after increasing the pressure but before decreasing the pressure.
In another embodiment, the invention features a method of detecting in a sample the presence of a nucleic acid that hybridizes to a reference nucleic acid at a first higher pressure but not at a second lower pressure by (1) providing a sample vessel and pressure controller for the vessel; and in any order (2) contacting the reference sequence with the sample in the vessel at the first pressure; (3) contacting the reference sequence with the sample in the pressure vessel at the second pressure; and (4) detecting the presence of a nucleic acid that hybridizes to the reference nucleic acid at the first pressure but not at the second pressure. In one aspect, the reference sequence is first contacted with the sample and hybridization is detected, and then the pressure is lowered and the absence of hybridization is detected.
The invention also features a method of discriminating between a first nucleic acid and a second nucleic acid that is different from the first nucleic acid by (1) providing a sample vessel and pressure controller for the vessel; (2) maintaining the vessel at a constant pressure; (3) providing the first and second nucleic acid and a reference nucleic acid in the vessel under conditions that do not allow either the first or the second nucleic acid to hybridize to the reference nucleic acid; (4) perturbing at least one condition (e.g., temperature or an electric field) to establish conditions that permit the first nucleic acid to form a complex with the reference nucleic acid at equilibrium and to permit the second nucleic acid to form a complex with the reference nucleic acid at equilibrium; and (5) comparing the time necessary to achieve equilibrium hybridization between the first nucleic acid and the reference nucleic acid with the time necessary to achieve equilibrium hybridization between the second nucleic acid and the reference nucleic acid, wherein the difference indicates the relative difference in sequence between the first and the second nucleic acids.
In addition, the invention features a method of discriminating between a first nucleic acid and a second nucleic acid that is different from the first nucleic acid by (1) providing a sample vessel and pressure controller for the vessel; (3) providing the first and second nucleic acid and a reference nucleic acid in the vessel under a first pressure that does not allow either the first or the second nucleic acid to hybridize to the reference nucleic acid; (4) perturbing the pressure to establish conditions that permit the first nucleic acid to form a complex with the reference nucleic acid at equilibrium and to permit the second nucleic acid to form a complex with the reference nucleic acid at equilibrium; and (5) comparing the time necessary to achieve equilibrium hybridization between the first nucleic acid and the reference nucleic acid with the time necessary to achieve equilibrium hybridization between the second nucleic acid and the reference nucleic acid, wherein the difference indicates the relative difference in sequence between the first and the second nucleic acids.
Methods of placing nucleic acids and optionally enzymes under pressure are described in WO 96/27432.
To facilitate understanding of the invention, a number of terms are defined below.
As used herein, the term xe2x80x9csolutionxe2x80x9d refers to a liquid, and more particularly, the incorporation of substances (e.g. dissolved compounds) in a liquid. The term xe2x80x9caqueous solutionxe2x80x9d refers to a solution either containing water or that is like water. For example, the present invention contemplates the use of assay buffers that are aqueous solutions.
As used herein, the term xe2x80x9cvectorxe2x80x9d is used in reference to nucleic acid molecules that transfer DNA segment(s) from one cell to another. The term xe2x80x9cvehiclexe2x80x9d is sometimes used interchangeably with xe2x80x9cvector.xe2x80x9d
The term xe2x80x9cexpression vectorxe2x80x9d as used herein refers to a recombinant DNA molecule containing a desired coding sequence and appropriate nucleic acid sequences necessary for the expression of the operably linked coding sequence in a particular host organism. Nucleic acid sequences necessary for expression in prokaryotes usually include a promoter, an operator (optional), and a ribosome binding site, often along with other sequences. Eukaryotic cells are known to utilize promoters, enhancers, and termination and polyadenylation signals.
As used herein, the terms xe2x80x9ccomplementaryxe2x80x9d or xe2x80x9ccomplementarityxe2x80x9d are used in reference to polynucleotides (i.e., a sequence of nucleotides) related by the base-pairing rules. For example, for the sequence xe2x80x9cA-G-T,xe2x80x9d is complementary to the sequence xe2x80x9cT-C-A.xe2x80x9d Complementarity may be xe2x80x9cpartial,xe2x80x9d in which only some of the nucleic acids"" bases are matched according to the base pairing rules. Or, there may be xe2x80x9ccompletexe2x80x9d or xe2x80x9ctotalxe2x80x9d complementarity between the nucleic acids. The degree of complementarity between nucleic acid strands has significant effects on the efficiency and strength of hybridization between nucleic acid strands. This is of particular importance in amplification reactions, as well as detection methods which depend upon binding between nucleic acids.
As used herein, the term xe2x80x9chybridizationxe2x80x9d is used in reference to the pairing of complementary nucleic acids. Hybridization and the strength of hybridization (i.e., the strength of the association between the nucleic acids) is impacted by such factors as the degree of complementary between the nucleic acids, stringency of the conditions involved, the Tm of the formed hybrid, and the G:C ratio within the nucleic acids.
As used herein, the term xe2x80x9cTmxe2x80x9d is used in reference to the xe2x80x9cmelting temperature.xe2x80x9d The melting temperature is the temperature at which a population of double-stranded nucleic acid molecules becomes half dissociated into single strands. The equation for calculating the Tm of nucleic acids is well known in the art. As indicated by standard references, a simple estimate of the Tm value may be calculated by the equation: Tm=81.5+0.41 (% G+C), when a nucleic acid is in aqueous solution at 1 M NaCl (see e.g., Anderson and Young, Quantitative Filter Hybridization, in Nucleic Acid Hybridization (1985). Other references include more sophisticated computations which take structural as well as sequence characteristics into account for the calculation of Tm.
As used herein the term xe2x80x9cstringencyxe2x80x9d is used in reference to the conditions of temperature, ionic strength, and the presence of other compounds such as organic solvents, under which nucleic acid hybridizations are conducted. With xe2x80x9chigh stringencyxe2x80x9d conditions, nucleic acid base pairing will occur only between nucleic acid fragments that have a high frequency of complementary base sequences. Thus, conditions of xe2x80x9cweakxe2x80x9d or xe2x80x9clowxe2x80x9d stringency are often required with nucleic acids that are derived from organisms that are genetically diverse, as the frequency of complementary sequences is usually less.
As used herein, the terms xe2x80x9cnucleic acidxe2x80x9d and xe2x80x9cnucleic acid substratexe2x80x9d encompass both DNA and RNA, whether single or double stranded. It is not intended that the present invention be limited by the length of the nucleic acid; the nucleic acid may be genomic or a defined length (e.g. short oligonucleotides) or fragments thereof (including single bases). It is also not intended that the present invention be limited by the nature or source of the nucleic acid. It may be naturally occurring, purified, produced synthetically, recombinantly or by amplification. The term xe2x80x9cmodified nucleic acid substratexe2x80x9d refers to the alteration of the structure of the nucleic acid substrate. To illustrate, an enzyme may remove a nucleotide from the nucleic acid substrate, yielding a modified nucleic acid substrate.
As used herein, the term xe2x80x9camplifiable nucleic acidxe2x80x9d is used in reference to nucleic acids which may be amplified by any amplification method. It is contemplated that xe2x80x9camplifiable nucleic acidxe2x80x9d will usually comprise xe2x80x9csample template.xe2x80x9d
As used herein, the term xe2x80x9csample templatexe2x80x9d refers to nucleic acid originating from a sample. In contrast, xe2x80x9cbackground templatexe2x80x9d is used in reference to nucleic acid other than sample template which may or may not be present in a sample. Background template is most often inadvertent. It may be the result of carryover, or it may be due to the presence of nucleic acid contaminants sought to be purified away from the sample. For example, nucleic acids from organisms other than those to be detected may be present as background in a test sample.
As used herein, the term xe2x80x9csamplexe2x80x9d is used in its broadest sense. That is, the term may encompass a specimen, culture, biological sample, environmental sample, etc. The term includes human and animal samples as well as naturally occurring and synthetic material. The term xe2x80x9csample vesselxe2x80x9d is used to indicate a means for containing a sample, whether by enclosing a sample (e.g., in a batch format) or by using a sample in an enclosed device (e.g., channeling both within and between a chamber, channel or stream). Similarly, a xe2x80x9creaction vesselxe2x80x9d is not limited to any one design; typically it is a sample vessel in which a reaction takes place.
A xe2x80x9creaction mixturexe2x80x9d refers to the mixing together of two or more components such that a reaction will occur. The term xe2x80x9coptimum enzymatic temperaturesxe2x80x9d refers to those temperatures at which an enzyme is most active. Thus, it depends on the characteristics of the individual enzyme. Typically, the optimum is between approximately 10 and approximately 80 degrees Centigrade, and more particularly between approximately 25 and 37 degrees Centigrade.
As used herein, the term xe2x80x9cinhibitionxe2x80x9d of enzyme activity be pressure refers to an enzyme that is reversibly inhibited at a particular pressure. When the pressure is altered (for example, lowered), the enzyme resumes its level of original activity.
As used herein the terms xe2x80x9csubstantially inactivexe2x80x9d and xe2x80x9crendering substantially inactivexe2x80x9d refer to an enzyme that exhibits less than approximately 20%, and generally less than 10%, of its activity at optimum enzymatic temperature (100% activity). An enzyme that has been rendered substantially inactive may also exhibit no activity, but may be reversibly inhibited such that activity of approximately normal levels returns under appropriate conditions of pressure, temperature and the like.
As used herein, the term xe2x80x9cprimerxe2x80x9d refers to an oligonucleotide, whether occurring naturally as in a purified restriction digest or produced synthetically, which is capable of acting as a point of initiation of synthesis when placed under conditions in which synthesis of a primer extension produce which is complementary to a nucleic acid strand is induced, (i.e., in the presence of nucleotides and an inducing agent such as DNA polymerase and at a suitable temperature and pH). The primer is preferably single stranded for maximum efficiency in amplification, but may alternatively be double stranded. If double stranded, the primer is first treated to separate its strands before being used to prepare extension products. Preferably, the primer is an oligodeoxyribonucleotide. The primer must be sufficiently long to prime the synthesis of extension products in the presence of the inducing agent. The exact lengths of the primers will depend on many factors, including temperature, source of primer and the use of the method.
As used herein, the term xe2x80x9cprobexe2x80x9d refers to an oligonucleotide (i.e., a sequence of nucleotides), whether occurring naturally as in a purified restriction digest or produced synthetically, which is capable of hybridizing to another oligonucleotide of interest. Probes are useful in the detection, identification and isolation of particular gene sequences. It is contemplated that any probe used in the present invention will be labelled with any xe2x80x9creporter molecule,xe2x80x9d so that is detectable in any detection system, including, but not limited to enzyme (e.g., ELISA, as well as enzyme-based histochemical assays), fluorescent, radioactive, and luminescent systems. It is further contemplated that the oligonucleotide of interest (i.e., to be detected) will be labelled with a reporter molecule. It is also contemplated that both the probe and oligonucleotide of interest will be labelled. It is not intended that the present invention be limited to any particular detection system or label.
As used herein, the term xe2x80x9ctargetxe2x80x9d refers to the region of nucleic acid bounded by the primers used for detection and/or amplification (e.g., by the polymerase chain reaction). Thus, the xe2x80x9ctargetxe2x80x9d is sought to be sorted out from other nucleic acid sequences. A xe2x80x9csegmentxe2x80x9d is defined as a region of nucleic acid within the target sequence.
As used herein, the term xe2x80x9cpolymerase chain reactionxe2x80x9d (xe2x80x9cPCRxe2x80x9d) refers to the method of K. B. Mullis U.S. Pat. Nos. 4,683,195 and 4,683,202, hereby incorporated by reference, which describe a method for increasing the concentration of a segment of a target sequence in a mixture of genomic DNA without cloning or purification.
As used herein, the terms xe2x80x9cPCR productxe2x80x9d and xe2x80x9camplification productxe2x80x9d refer to the resultant mixture of compounds after two or more cycles of the PCR steps of denaturation, annealing and extension are complete. These terms encompass the case where there has been amplification of one or more segments of one or more target sequences.
As used herein, the term xe2x80x9camplification reagentsxe2x80x9d refers to those reagents (deoxyribonucleoside triphosphates, buffer, etc.), needed for amplification except for primers, nucleic acid template and the amplification enzyme. Typically, amplification reagents along with other reaction components are placed and contained in a reaction vessel (test tube, microwell, etc.).
As used herein, the terms xe2x80x9crestriction endonucleasesxe2x80x9d and xe2x80x9crestriction enzymesxe2x80x9d refer to bacterial enzymes, each of which cut double-stranded DNA at or near a specific nucleotide sequence.
As used herein, the term xe2x80x9crecombinant DNA moleculexe2x80x9d as used herein refers to a DNA molecule which is comprised of segments of DNA joined together by means of molecular biological techniques.
DNA molecules are said to have xe2x80x9c5xe2x80x2 endsxe2x80x9d and xe2x80x9c3xe2x80x2 endsxe2x80x9d because mononucleotides are reacted to make oligonucleotides in a manner such that the 5xe2x80x2 phosphate of one mononucleotide pentose ring is attached to the 3xe2x80x2 oxygen of its neighbor in one direction via a phosphodiester linkage. Therefore, an end of an oligonucleotides referred to as the xe2x80x9c5xe2x80x2 endxe2x80x9d if its 5xe2x80x2 phosphate is not linked to the 3xe2x80x2 oxygen of a mononucleotide pentose ring and as the xe2x80x9c3xe2x80x2 endxe2x80x9d if its 3xe2x80x2 oxygen is not linked to a 5xe2x80x2 phosphate of a subsequent mononucleotide pentose ring. As used herein, a nucleic acid sequence, even if internal to a larger oligonucleotide, also may be said to have 5xe2x80x2 and 3xe2x80x2 ends. In either a linear or circular DNA molecule, discrete elements are referred to as being xe2x80x9cupstreamxe2x80x9d or 5xe2x80x2 of the xe2x80x9cdownstreamxe2x80x9d or 3xe2x80x2 elements. This terminology reflects the fact that transcription proceeds in a 5xe2x80x2 to 3xe2x80x2 fashion along the DNA strand. The promoter and enhancer elements which direct transcription of a linked gene are generally located 5xe2x80x2 or upstream of the coding region. However, enhancer elements can exert their effect even when located 3xe2x80x2 of the promoter element and the coding region. Transcription termination and polyadenylation signals are located 3xe2x80x2 or downstream of the coding region.
As used herein, the term xe2x80x9can oligonucleotide having a nucleotide sequence encoding a genexe2x80x9d means a DNA sequence comprising the coding region of a gene or in other words the DNA sequence which encodes a gene product. The coding region may be present in either a cDNA or genomic DNA form. Suitable control elements such as enhancers/promoters, splice junctions, polyadenylation signals, etc., may be placed in close proximity to the coding region of the gene if needed to permit proper initiation of transcription and/or correct processing of the primary RNA transcript. Alternatively, the coding region utilized in the expression vectors of the present invention may contain endogenous enhancers/promoters, splice junctions, intervening sequences, polyadenylation signals, etc., or a combination of both endogenous and exogenous control elements.
As used herein, the terms xe2x80x9cnucleic acid molecule encoding,xe2x80x9d xe2x80x9cDNA sequence encoding,xe2x80x9d and xe2x80x9cDNA encodingxe2x80x9d refer to the order or sequence of deoxyribonucleotides along a strand of deoxyribonucleic acid. The order of these deoxyribonucleotides determines the order of amino acids along the polypeptide (protein) chain. The DNA sequence thus codes for the amino acid sequence.
Enzymes that synthesize polymers may dissociate after each catalytic event, i.e., they may be xe2x80x9cnonprocessive.xe2x80x9d On the other hand, they may remain bound to the polymer until many cycles of reaction are completed, i.e., they may be xe2x80x9cprocessive.xe2x80x9d See A. Kornberg, DNA Replication (Freeman and Co. 1980).
Other features or advantages of the present invention will be apparent from the following detailed description and from the claims.
The invention is based on the realization that the molar volumes of mismatched nucleic acid hybridization products are larger than the molar volumes for complementary hybridization products. Thus, high pressure will tend to drive the hybridization reaction equilibrium towards correct matches and away from mismatches and result in a higher signal to noise ratio in a given hybridization. Higher temperatures may also-be employed for a further decrease in mismatching. Dissociation of fully complementary nucleic acids can be slowed by high pressure, whereas dissociation of mismatched sequences can be enhanced or slowed to a lesser degree than fully complementary hybrids. Pressure effects on melting and dissociation may provide information that allows assignment of a mismatched base with fewer than four probes or may increases the sensitivity of a hybridization reaction.
Modulation of hydrostatic pressure has multiple advantages in hybridization assays. Pressure allows rapid modulation of the stringency of hybridization. By raising the melting temperature of hybrids, higher hybridization temperatures may be used with the advantage of more rapid approach to equilibrium. Pressure differs from other forms of stringency modulation such as salt concentration or temperature in that it provides additional discrimination between match and mismatch. This is of particular advantage when the base to be discriminated is at or near the terminus of one of the sequences to be assayed. The additional sensitivity provided by increased specificity under pressure allows for multiple benefits, such as more rapid data collection, higher density arrays, and shorter hybridization times. Pressure operates without exchange of fluids and may be transmitted at up to the speed of sound.
Several devices may be used for the application of hydrostatic pressure to hybridization reactions, arrays or microchip-based hybridization arrays. The reaction may be sealed in a deformable container and the container is placed in a device that generates elevated hydrostatic pressures and is temperature controlled. After the hyperbaric hybridization reaction is complete, the reaction is removed from the pressurizing apparatus and analyzed by the various means known in the art.
Another useful device is a hyperbaric chamber with an optical window. Such devices are known in the art of high pressure science and include the diamond anvil cell, the sapphire anvil cell and fiber-optic feed-throughs into hyperbaric chambers (Eremets, M xe2x80x9cHigh Pressure Experimental Methodsxe2x80x9d, 1996, Oxford University Press: New York). With this device, the effects of changing the pressure or temperature may be measured continuously. For example, the rate of strand dissociation in a hybridization array may be measured after a rapid pressure drop to yield sequence information. This experiment may make use of the knowledge that pressure reduces the rate of hybrid melting so that by raising the pressure and then the temperature, one can put the system in a state of non-equilibrium which is high in potential energy. The pressure is then released and the rate of strand melting is measured.
Alternatively, a hybridization reaction may be conducted at low effective stringency by being conducted at high pressure. The pattern of hybridization to a defined polynucleotide array is determined by optical or other means. The pressure is then reduced to lower the stringency, and the change in the pattern of bound DNA is again determined, optionally after flushing away nucleotides which were released. Further cycles of pressure lowering and optional flushing allow determination of the degree of affinity of the bound DNA to the array.
In a method of the invention, the hybridization reaction is allowed to come to equilibrium while under stringent conditions at an elevated hydrostatic pressure, such as 100,000 psi. The reaction is then washed with a mildly stringent buffer and the hybridization pattern is determined.
In another method of the invention, the hybridization reaction occurs under conditions of low stringency and then is washed under conditions of high stringency while under pressure. A device suitable for this purpose is (xe2x80x9cVersion3xe2x80x9d) described in WO 96/27432. Other devices such as those utilizing electrophoresis or electroosmosis may be used. The washing solution may be held under pressure and then rapidly replaced with a non-stringent solution at atmospheric pressure, provided the exchange is rapid enough that the system does not significantly return to a condition of reduced selectivity.
The extent of sequence-specific hybridization is determined by measurement of the rate of relaxation toward equilibrium following a perturbation such as a rapid temperature jump at several hydrostatic pressures. The pressure effect on the rate of approach to equilibrium will be a sensitive discriminator of matched vs. mismatched sequences and will allow for a well-defined and more easily measured dissociation rate. In addition, singly and multiply mismatched sequences will be more easily determined. The measured dissociation rate at high pressure may be sufficient to accurately discriminate between mismatches.
Another perturbation which may be used in this technique is electrical perturbation as described in U.S. Pat. Nos. 5,632,957 and 5,605,662 and involves monitoring changes in fluorescence while short pulses of electric field perturbations are applied.
Since the methods of the invention results in increased specificity, sensitivity, or selectivity of hybridization, such methods can be used to reduce mis-primings in technological applications involving polymerase or ligase enzymes, which require hybridization of a primer to a template. Examples include PCR, Sanger sequencing reactions, and LCR. Hybridizations may be done under high pressures such as 10,000 psi or 100,000 psi. It is possible to find a pressure at which the optimal hybridization temperature is the same as the optimal temperature for polymerase extension, thereby decreasing the number of thermal cycling steps needed for any suitable application. In addition, it is anticipated that fewer errors will result in polymerase extension reactions at elevated pressures the specificity of the editing mechanism is increased due to increased exonuclease activity. Optimal pressures for hybridization and extension steps may differ and are expected to be in the range of 5000 psi to 100,000 psi. High pressures have the added advantage of further stabilizing the thermostable enzymes which are often used in these reactions.
Improved Biochip Array Performance
An important application for the techniques and apparatus of the invention is in the improved speed and accuracy of readout in xe2x80x9csequencing by hybridizationxe2x80x9d and in the closely related, currently-commercial techniques applied to gene identification and diagnosis. In these techniques, an array of single-stranded nucleic acidsxe2x80x94usually DNA, or less commonly RNA, and in principle including non-natural nucleic acids such as xe2x80x9cpeptidexe2x80x9d nucleic acidsxe2x80x94is synthesized on the surface of a xe2x80x9cchipxe2x80x9d, of silicon, silica/glass, or other planar material. Techniques derived from microelectronicsxe2x80x94in particular, sequential use of masks followed by chemical development, sometimes optically stimulatedxe2x80x94allow the simultaneous synthesis of differing sequences of nucleic acids in different areas of the same chip. (The principles are extensively described in the literature, and are known in the art.) The resultant pattern can be used for gene diagnosis, or for sequencing of DNA.
To use the nucleic acid chips, the nucleic acids to be tested are labeled in any known manner, typically with one or more fluorescent labels. The nucleic acids are then rendered single-stranded, if necessary, and allowed to hybridize with the single stranded nucleic acids on the chip. The pattern of hybridization allows at least partial determination of the sequence of the tested nucleic acid, since the sequence of the areas of the chip to which the tested nucleic acids hybridized are known.
However, mismatches during hybridization are possible. For example, a chip sequence of 8 nucleotides will bind tested sequences in which 8, 7, 6, or even 5 or less of the bases within the nucleic acid are properly base-paired, depending on the xe2x80x9cstringencyxe2x80x9d of the hybridization conditions. The concept of xe2x80x9cstringencyxe2x80x9d is well-known in the art. In essence, when the conditions in the test solution (e.g., salt concentration, buffer composition, pH and temperature) favor binding, the hybridization conditions are described as xe2x80x9cless stringentxe2x80x9d or xe2x80x9cof low stringencyxe2x80x9d. When conditions are less favorable to hybridization, then only well-matched sequences will hybridize, and the conditions are described as xe2x80x9cstringentxe2x80x9d, or xe2x80x9cmore stringentxe2x80x9d. Under ideal stringency, only the target sequence of interest (e.g., a perfectly complimentary sequence) will hybridize to a reference nucleic acid.
In diagnosis of genetic shifts, if a sample of DNA is restricted, labeled, and rendered single-stranded, then with an ordered array of nucleic acids on the test chip, and at high stringency, only sequences specific to the various alleles of the gene in question will bind. However, in actual situations, and in particular when amplification means are not used, many different restriction fragments of the genome will have some affinity for the array, and at low stringency, binding of test nucleic acid to most portions of the array will occur. On the other hand, at high stringency, hybridization will be slow even for well-matched nucleic acids, and may tend to be incomplete, resulting in loss of signal.
Therefore, as a practical matter, such tests are usually initiated at low stringency, where reactions are fast and quantitative, and then the stringency is gradually increased by increased temperature and/or lower ionic strength, and at the end of the process only highly homologous nucleic acids are still hybridized to the chip. The last nucleic acids to dissociate are normally the best matched to that particular area of the chip, and therefore are judged to be substantially similar to the target sequence.
While these procedures can be effective, they are also slow. With typicaly hybridization times of many minutes for each step of increased stringency, arriving at optimal conditions can take up to a few hours. The essence of the invention is the application of control of hybridization stringency by pressure. As noted above, pressure changes are very rapid, and have a characteristic time of equilibration of less than a microsecond in small volumes. One can thus arrange a continuous or pulsed flow of buffer of constant composition, and conduct the stringency analysis by decreasing the pressure. The speed of successive steps of increased stringency is limited only by the time required for less stringently bound nucleic acids to dissociate from the reference nucleic acid. Moreover, the speed of hybridization is increased at higher pressure, so the initial low-stringency hybridization step is also faster. As noted above, the time required for dehybridization, as well as any optical or other signals generated by dehybridization, may also be a factor in sequence determination.
Because the capital cost of the sequencing-by-hybridization equipment is a significant portion of the cost of performing such an assay, the use of pressure changes to control hybridization (in place of the usual temperature and/or buffer change) can increase the speed and accuracy of the analysis, which in turn can significantly lower the cost of such assays.
A system to perform such pressure-hybridization assays is also a part of the invention. The mechanical portions of such a device have previously been described in WO 96/27432. The device includes a pressurizable chamber with means for rapid pressure changes between atmospheric and a higher pressure (such as 5,000, 10,000, or 50,000 psi), and means for introduction and removal of fluids. The device is adapted to improve sequencing-by-hybridization by inclusion of a chip or slide bearing defined immobilized nucleic acid sequences in known areas, and a flow arrangement which efficiently washes the nucleic acid-bearing surface(s) of the chip with a buffer, to remove unhybridized nucleic acids, and a means for detecting the extent of dehybridization and the pattern of remaining hybridization on the chip.
The detection means may be any means known in the art which are compatible with the high pressures and sudden pressure changes necessary for the assays. The currently-preferred means for detection of hybridized test DNA is a fluorescent label, and the detection is accomplished by direct visualization. To use this system, the pressurizable chamber is provided with an optical window which allows accurate projection of emitted fluorescent light onto a detection means, such as a video camera or a photodiode array. A suitable means is an imaging lens embedded in the wall or end of the pressurizable reaction volume, which images the surface of the chip to an outside detection means. An intermediate adaptable lens, with position and/or optical power controlled electronically, and preferably located outside the pressurizable zone, can be used to correct distortions arising from changes in pressure on the imaging lens. Another option is a flat sapphire window, with an adjustably-positioned detection device, as is known from the literature related to the analytical ultracentrifuge.
Alternatively, the detection means could be a light-sensitive diode array mounted behind the nucleic acid array-bearing chip, baffled so that fluorescent light emitted from a defined area of the array falls on a particular diode or diodes, with a suitable pressure-resistant cable for conduction signals to an outside display and recording means, such as a computer.
As noted above, the effects of pressure will be beneficial in performing various sorts of nucleic acid amplification assays. These are described here in terms of some well-known DNA amplification methods, and are also applicable to amplification of other types of nucleic acids.
Nucleic Acid Amplification under High Pressure
Polymerase Chain Reaction (PCR).
In this widely practiced method, a particular sequence of DNA, typically a restriction fragment, is amplified by use of a primer, or preferably two primers, and a DNA polymerase, in the presence of four deoxyribonucleoside triphosphates (dNTPs). The primer is allowed to hybridize, at a first hybridization-permissive temperature, to a stretch of ssDNA to be amplified. Then the polymerase, which requires a short sequence of double-stranded DNA as a xe2x80x9cstartxe2x80x9d point, will proceed along the DNA to make a copy of it. If there are two primers, then amplification proceeds on both strands. Next, the reaction mixture is heated to dissociate the newly-formed copies from the original. On cooling, new primers hybridize, and the cycle is repeated. In principle, the numbers of copies of the target DNA is doubled in each cycle, leading to a large increase in the amount of the target DNA sequence.
The amplified copies can be fluorescently labelled. If di-deoxy nucleotides are used in the last few cycles, then the resultant mixture can be used for Sanger-type sequencing.
High pressure offers three distinct avenues for improvement of the PCR.
First, it can be used to replace the temperature-cycling step with a pressure controlled dehybridization/hybridization cycle. This is inherently faster and allows a xe2x80x9cquick-startxe2x80x9d procedure. In the high-pressure-PCR (HP-PCR) assay, the polymerase""s sensitivity to pressure is determined, and a first pressure P3 is selected. P3 is low enough to retain enzymic activity and high enough to enable primer binding. A temperature T1 is selected at which the enzyme can function, and at which the primers (and primer extension products) will be dehybridized from the test DNA at low pressure. Typically, enzymes that are thermostable have enhanced tolerance to hyperbaric conditions. This temperature (T1) can be predicted from first principles and the primer sequence composition, and quickly optimized to the actual situation.
The reaction mixture is prepared at an initial P1/T1 state, except for the polymerase, and is raised to the P3/T1 state. The primers hybridize to the template. Enzyme is added under pressure and the primers are extended at a P2/T1, where P2 can be between P1 and P3. After the new strands have been polymerized, the pressure is dropped to P1 at which the primers and extension products will dehybridize. The pressure is raised back to P3/T1 and held briefly to allow primer hybridization/extension before progressing to P2/T1 for primer extension. Then pressure is reduced to P1/T1, and the cycle is repeated. The consequences at the P1/T1, P2/T1, and P3/T1 states are summarized below.
The reactions immediately above were conducted at one temperature, T1, to illustrate one example of the methods of this invention. However, performing reactions at different temperatures in conjunction with different pressures can be ideal under some circumstances.
Because pressure changes propagate through the medium at the speed of sound, and require no convection or mixing, pressure changes can occur throughout a medium much quicker than temperature changes. Consequently, the reaction medium spends more time at equilibrium. Therefore, the time required to conduct a PCR reaction is reduced. Since PCR is important in genome sequencing, and increasing in forensics and diagnostics, the decrease in time required is advantageous.
Second, careful selection of temperatures and pressures can increase the performance of an enzyme. As noted above, in many cases it will be possible to find a temperature and pressure combination at which the activity of the enzyme is optimized, which will shorten the time required in the polymerization step. This speed improvement is independent of the speed improvement obtained by pressure changes. In addition, by selection of temperature and pressure, the stability of the enzyme against denaturation or other inactivation can be enhanced. In particular, the same optimum pressure for activity will normally suffice to prevent enzyme denaturation, because the additional cycling step involves a decrease in pressure (disfavoring enzyme denaturation) rather than an increase in temperature (favoring enzyme denaturation). This may allow additional kinds of polymerases to be used in PCR.
Moreover, at some combinations of temperature and pressure, the polymerase will be more tightly bound to the DNA being replicated, and is expected to exhibit increased processivity, resulting in longer strands on average, and a higher proportion of fully replicated strands in the mixture. This is critical when using PCR to amplify long stretches of DNA, for example 1000 bases or more.
Third, running the PCR at high pressure is especially beneficial because the effective stringency of the reaction can be improved at high pressure. Improvement of stringency is described above in the discussion of xe2x80x9cDNA chipsxe2x80x9d. Because pressure widens the temperature spread between hybridization of matched and mismatched primers, binding of primers to mismatched sites can be significantly reduced at high pressure compared to ambient pressure. The speed of change between dehybridizing and hybridizing conditions can also diminish the proportion of mismatched hybridizations in the mixture. In turn, there will be fewer errors in the amplified sequences, resulting in better quality (via higher fidelity of polymerization) of the amplified DNA for its intended use, such as for sequencing, or as a probe for use in a reaction involving a DNA chip.
In addition, the primers can be shorter in a high-pressure reaction, because formation of double-stranded regions is favored at high pressure. Current practice favors longer primers both for specificity and sensitivity of hybridization. With pressure, and particularly when specificity within the genome is less critical, primers as short as eight to ten nucleotides can be used.
Ligase Chain Reaction (LCR).
In LCR, the two primers are directed to the same strand, and are contiguous. If a target strand is present in the DNA to be tested, both primers hybridize to adjacent sequences on the target. A DNA ligase is added to join the strands. The ligated product is dissociated by an increase in temperature, and the process repeats after cooling. The presence of ligated DNA is detected by any of several means (e.g., molecular weight or fluorescence energy transfer).
Essentially all of the advantages of pressure cycling in the PCR apply to the LCR. The increased selectivity for accurately matched probes is particularly critical in LCR, since it is less sensitive than the PCR to single-base substitutions and deletions, especially away from the ligation site.
Other Amplification Methods.
There are now over a dozen other methods of nucleic acid amplification. Most of these require a hybridization/dehybridization cycle. The use of pressure cycling as a replacement for temperature cycling will be feasible in any such amplification reaction.
Without further elaboration, it is believed that one skilled in the art can, based on the above disclosure and the examples below, utilize the present invention at its fullest extent. The following examples are to be construed as merely illustrative of how one skilled in the art can practice the invention and are not limitative of the remainder of the disclosure in any way. Any publications cited in this disclosure are hereby incorporated by reference.